Our long-term objective is to understand the role of vascular endothelium in the pathogenesis of auto-immune insulin-dependent diabetes mellitus (IDDM). Expression of IDDM depends on many factors that we view as analogous to tumblers in a lock. These include the target beta cell, effector and regulatory immunocytes, humoral factors, genetics, environment, and pancreatic endothelial cells (EC). Work performed during the previous grant period has increased our understanding of the endothelial tumbler. We now know that the pancreatic endothelium of rats susceptible to IDDM is prone to abnormal leakiness, in part dependent on a prostaglandin mediated interaction between the EC and macrophages. We have also shown that effector T cells and EC interact in a costimulatory fashion before there is morphological evidence of insulitis, and recently we discovered that anti-EC antibodies capable of inducing abnormal permeability are also generated in advance of insulitis. We hypothesize that EC dysfunction contributes to the pathogenesis of IDDM by disrupting vascular integrity, by enhancing mononuclear cell adherence, or by generating inflammatory cytokines. This hypothesis will be tested in the RT6-depleted DR rat, a well characterized animal model of human IDDM. Specific Aim #1 is to determine in vitro the mechanisms by which EC-T cell interactions lead to EC dysfunction. We will identify the antigens induced on pancreatic EC, analyze EC interaction with relevant T cell subsets, and seek to identify T cell populations capable of suppressing EC activation. Specific Aim #2 is to evaluate the hypothesis that anti-pancreatic EC antibodies contribute to IDDM. We already know that anti-EC antibodies not only precede the onset of insulitis but can also induce abnormal pancreatic vascular leakiness. The biochemical and physiological properties of these antibodies, as well as their in vivo an in vitro activities, will be defined. Specific Aim #3 is to identify in vivo the mechanisms by which EC dysfunction contributes to IDDM pathogenesis in the BB rat. This aim is premised on the fact that BB rat EC leak abnormally and are the target of autoantibodies. We will emphasize electron microscopic analysis of EC pathology and quantitative measurements of EC activation. Our ultimate goal is to understand the importance of the pancreatic endothelium as an initiator or abettor of IDDM. The results of these studies should define the critical role of islet EC in diabetes pathogenesis, uncover the mechanisms by which EC dysfunction contributes to the progression of islet pathology, and suggest what therapies might arrest the process and ultimately prevent the disease.